Process and composition for phosphatizing steel



PROCESS AND CUMPQSETIQN FOR PHOSPHATIZENG STEEL Daniel E. Miller, East Los Angelcs, alit., assignor to Kelite Products, Inc., Los Angeies, aii., a corporation of California No Drawing. Application January E1, 1954 Serial No. 463,451

15 Claims. (Cl. 14$-d.l5)

compounds are produced which plate out on the metal to be protected in the form of a complex coating. A coating is desired which will neither rub off nor crack off, which will exhibit high corrosion inhibiting properties, and form a satisfactory bond with paints, lacquers and enamels.

Standards are established in the art, such as conical b mandrel bend test and salt-fog tests for testing the coatings formed by the phosphatizing process.

It is a general object of the present invention to provide a process and composition for phosphatizing metal which will provide a coating equal in properties and, in some forms of the invention, superior in properties to those now employed, which process and compositions allow a reduction in the time necessary to immerse the steel to be coated in the composition, which process and compositions permit the phosphatizing of steel at a lower cost by the use of a solution extremely dilute as compared with prior processes.

The process and compositions of the present invention differentiate from those now in use in the employment of acid pyrophosphates. It has been previously believed that such metal pyrophosphate compounds as zinc and manganese pyrophosphate are essentially insoluble compounds. I have discovered, however, that it is possible to maintain iron, zinc, manganese and molybdenum compound in a phosphatizing solution which contains pyrophosphate, and as a result of including pyrophosphate in such a solution it is possible to secure better coatings more rapidly and at a lower cost. By the introduction of pyrophosphate into the phosphatizing solution the solubility of the zinc and manganese is lower than when merely orthophosphates are employed but the zinc and manganese plate out more rapidly from the more dilute solution providing superior coatings as well as reducing the time and cost of treatment.

When an acid solution of Zinc pyrophosphate and/or manganese pyrophosphate is brought into contact with steel, the solution reacts to bring iron into solution with the resulting lowering in the solubility of zinc pyrophosphates and manganese pyrophosphate. This causes a deposition onto the metalto be phosphatized to form the coating. With this deposit there is an accompanying liberation of two hydrogens per mol of either Zinc or manganese pyrophosphate deposited. This liberation of hydrogen serves to maintain the pH of the phosphatizing solution at the desired level.

With the compositions of the present invention at the atom is first plated Zinc pyrophosphate ZllzPzOq onto the metal to be treated. Later as the solution dissolves the iron there may be plated a complex of zinc, iron, pyrophosphate, such as ZnFeP O Where manganese .is included in the phosphatizing solution manganese pyrophosphate may also be plated out onto the metal, or a complex of zinc, manganese or iron pyrophosphate. As the useful supply of pyrophosphates in the solution becomes expended the solution may start then to plate out zinc and iron orthophosphates. At this step in the process there generally appears a cloudy precipitate in the solution which serves as a useful indicator that the solution should be replaced.

Essentially the phosphatizing process of the present invention is carried out employing a solution containing zinc, sodium pyrophosphate and free phosphoric acid. The zinc is most conveniently added in the form of Zinc oxide ZnO but it will be recognized that in the solution there will be formed the zinc pyrophosphate and zinc acid phosphate.

Phosphatizing solutions containing only such ingredients are quite acid having a pH generally between 2 and 3, and require the use of stainless steel equipment conducting the phosphatizing process. Some advantage appears also to be gained Where stainless steel equipment is available in rendering the solution somewhat more active by the addition of small quantities of citric acid. My phosphatizing process is best cairied out with the phosphatizing solution at temperatures of between 60 and 82 C., and can be completed in the process of the present invention with an immersion time of the i cal to be phosphatized of about ten minutes. For the most satisfactory results there appears to be a definite ratio between the zinc content of the solution and the phos phoric acid in the solution free to combine therewith. Generally when the zinc content of the solution is calculated to ZnO the ratio should be between the limits of 1 part of ZnO to from 3 to 4.7 parts of phosphoric acid H PO When the Z110 content is greater than 1 to 3H PO there is a tendency for zinc pyroph-osphate to precipitate on heating with an nnecono-mical employment of the zinc. When the Z110 content is less than l part to 4.7H PO the quality of the coating on the phosphatized metal is of inferior grade. The sodium acid pyrophosphate Na H P O, content of the solution is preferably from 1 to 6 grams per liter of solution although higher concentrations up to, for example, 25 grams per liter may be employed with, however, no improvement in operation and attended with the danger that with high zinc concentrations in solution some zinc may be precipitated as zinc pyrophosphate with a loss of economy in the process.

The zinc concentration of the phosphatizing solution may be from 0.026 to as high as 2.5 grams ZnO per liter. However, the solubility of zinc pyrophosphate in solution decreases as the content of the sodium acid pyrophosphate per liter is increased. The sodium acid pyrophosphate content of the solution may vary between the limits of 1 and 25 grams per liter but i have found no advantage in maintaining more than 6 grams per liter of the sodium acid pyrophosphate in the solution. if more than 6 grams of sodium acid pyrophosphate per liter are included in the phosphatizing solution uneconomical precipitation of some zinc may occur if there is more than 0.6Zn0 per liter employed. Likewise, if the phosphatizing bath is made more alkaline as hereafter described, the concentration of zinc in the solution should also be diminished.

While in the method of the present invention it is only necessary to employ zinc, phosphoric acid and sodium acid pyrophosphate, particularly where stainless steel start of the phosphatizing process it appears'that there 1 equipment is available for the phosphatizing process,

Patented Mar. it, 1958 many other ingredients may be included in the phosphatizing bath and certain modifications ofthe phosphatizing composition are desirable where the equipment employed in the phosphatizing operation is not of stainless steel. Where other than stainless steel equipment is employed it is desirable to reduce the pH of the phosphatizing solution to around between 3 and 4 and for. this purpose the addition of sodium acidorthophosphates is recommended. The orthophosphates may be addedin the form of sodium phosphate and/ or hemi basic sodium.

phosphate and obviously equivalent results could be obtained but with less economy by adding caustic soda and additional phosphoric acid to provide sodium phosphate in the phosphatizing bath to regulate the pH of the solution.

The phosphatizing solution may also include manganese and molybdenum compounds. These are best added in the form of 85% molybdic oxide and manganese carbonate. The reduction of the acidity of the phosphatizing solution, by adding sodium phosphate reduces the solubility of zinc in the phosphatizing bath and in such cases the zinc concentration is usually maintained between the limits of 0.026 to 0.23% ZnO, and the manganese concentration limited to from zero to 0.17% Mn. It is also possible to include such ingredients as citric acid in the phosphatizing bath or equivalent sequestering agents. Citric acid has a property of sequestering or rendering more soluble zinc pyrophosphate and manganese concentration limited to from zero to. 0.17% Mn. sacrifice of the facility with which such complexes plate out on the metal in the desired phosphatizing operation so the amount of such ingredients to be included in the bath is preferably limited.

The method and compositions of the present invention together with various further objects and advantages of the invention will be understood from the following description of the preferred forms of the invention.

In the form of the invention as utilized with stainless steel equipment, the process is preferably carried out by preparing a concentrated solution of all the ingredients to be added to make the phosphatizing bath except the sodium acid pyrophosphate so that the desired phosphatizing bath is prepared by adding separately such concentrated solution and the sodium acid pyrophosphate. The preferred concentrated solution is one made employing the following ingredients in the following proportions:

Percent ZnO 10 H O 23 75% H PO 62 70% HNO 5 This concentrated solution is a stable commodity for shipping. In practice it is added to water to form a phosphatizing bath in proportions of to gallons to 1000 gallons of water, or in proportions of about 10 to 15 cc. per liter, providing a phosphatizing bath containing about 1.3 to 2 grams ZnO per liter. The sodium acid pyrophosphate is added in the proportion of lbs. per 1000 gallons of water or 2.5 grams of sodium acid pyrophosphate per liter. This preferred phosphatizing bath has a pH when freshly prepared of 2.1-2.2.

Such a phosphatizing bath is capable of effecting satisfactory phosphatizing of metals in about 10 minutes of immersing time at 70 C. As the zinc and pyrophosphate content of the bath are expended in phosphatizing steel progressive additions of further amounts of the concentrated solution and the sodium acid pyrophosphate may be made in order to maintain the proper concentration in the phosphatizing solution. Sometimes it is found more economical to dump the solution as the zinc and pyrophosphate concentration diminish with use rather than to replenish these by further additions.

Where stainless steel equipmentis not available I prefer the employment of a phosphatizing bath less acidic 4 and containing sodium orthophosphates. In such case it is convenient to provide all the ingredients required to form the phosphatizing bath as one single powdered admixture. The preferred powdered mixture for this purpose is produced by admixing the following ingredients in the proportions indicated:

Percent Zinc oxide, ZnO 0.70 H PO 4.8 Water l 85% inclybdic oxide, M00 0.5 (Anhy) mo-nosodium phosphate 32-.8 Manganese carbonate, MnCO 1.37 Sodium acid pyrophosphate 33.00 Citric acid 1.25 Hemi sodium pyrophosphate 25.00

With this composition I generally prefer to employ /2 to 4 ounces of the dry mixture per gallon, or from 3.7 grams to 30 grams of the dry powdered admixture per liter. The pH of this preferred phosphatizing solution at 1 oz./gal. is about 3.1.

The produced phosphatizing bath at 1 oz./gal. will then contain approximately .05 gram ZnO per liter, .05 gram Mn per liter, and 2.5 grams sodium acid pyrophosphate per liter. With such dry composition, however, from /2 to 4' ounces of the dry mix may be added per gallon so that the phosphatizing solution may contain from 0.026 to 0.21 gram ZnO per liter, .025 to 0.2 gram Mn per liter, and from 1.25 to 10 grams sodium acid pyrophosphate per liter. The dry composition contains 4.8% of 85% H PO This corresponds to 4.08% H PO The manganese carbonate reacts with some of the H PO to form manganese phosphate reducing the free H PO available for combination with the ZnO to 3.3% so that the ratio of ZnO to H PO in the phosphatizing bath is 1 part ZnO to 4.7 parts free H PO plus the free phosphoric acid of hemi sodium phosphate (at 45% of the hemi sodium phosphate) would be 11% more.

While in these two preferred compositions I have indicated the proportions which I have found most desirable in practice, the constituents may be employed.

in greater concentrations in the phosphatizing bath as heretofore indicated without, however, any benefits being derived therefrom. While either phosphatizing bath may i be employed for phosphatizing steel in the usual manner. by immersion therein, I have found that the phosphatizing solution I have described is sufficiently active for phosphatizing metals so that metals may be phosphatized' by spraying such solutions onto the metals.

Since pyrophosphoric acid is derived from orthophosphoric acid by molecular dehydration it might be thought that in solution sodium acid pyrophosphate would hydrate rapidly to the sodium acid orthophosphate. I have heated the phosphatizing bath of the present invention at temperatures of 70 C. for as high as four hours without any measurable loss of sodium acid pyrophosphate. At higher temperatures, such as 85 and there is a slight drop in one or two hours heating in the pyrophosphate content in the neighborhood of 10%. The phosphatizing solution of the present invention may be allowed to stand without use for a period of twentyfour hours or more without the pyrophosphate constituents being hydrated to orthophosphate sufficiently to interfere with the utility and process of the present invention.

The percentages referred to in this specification are percentages by weight unless otherwise specified.

While the particular examples of the process and compositions of the present invention herein described are well adapted to carry out the objects of the invention, this invention is of. the. scope set forth. in the appended claims.

I claim:

1. A process of phosphatizing metals which comprises, contacting the metal with a phosphatizing solution comprising zinc, free phosphoric acid and sodium acid pyrophosphate, the initial ratio of zinc to free phosphoric acid being in the proportion of from 1 part ZnO to 3 to 4.7 parts of H PO the zinc being present in an amount equivalent to 0.026 to 2.5 grams per liter, and the sodium acid pyrophosphate being present in amounts from 1 to 25 grams per liter said ingredients in the solution reacting to form zinc pyrophosphate and zinc acid phosphate, said solution being maintained at a temperature of from about 60 C. to about 82 C.

2. A process of phosphatizing metals which comprises, contacting the metal with a phosphatizing solution comprising zinc, free phosphoric acid and sodium acid pyrophosphate, the initial ratio of zinc to free phosphoric acid being in the proportion of from 1 part ZnO to 3 to 4.7 parts of H PO the zinc being present in an amount equivalent to 0.026 to 2.5 grams per liter, and the sodium acid pyrophosphate being present in amounts from 1 to 6 grams per liter said ingredients in the solution reacting to form zinc pyrophosphate and zinc acid phosphate, said solution being maintained at a temperature of from about 60 C. to about 82 C.

3. A process of phosphatizing metals which comprises, subjecting the metal to a phosphatizing solution comprising zinc, free phosphoric acid, sodium acid pyrophosphate and sufficient sodium orthophosphate to provide a pi-l of between 3 and 4, the free phosphoric acid being initially present in the proportion of 1 part ZnO to 3 to 4.7 parts H PO the zinc being present in amounts equivalent to from 0.026 to 0.21 gram ZnO per liter, the sodium acid pyrophosphate being present in an amount equivalent to from 1 to 6 grams per liter said ingredients in the solution reacting to form zinc pyrophosphate and zinc acid phosphate, said solution being maintained at a temperature of from about 60 C. to about 82 C.

4. A process of phosphatizing metals which comprises, subjecting the metal to a phosphatizing solution comprising zinc, free phosphoric acid, sodium acid pyrophosphate and suflicient sodium orthophosphate to provide a pH of between 3 and 4, the free phosphoric acid being initially present in the proportion of 1 part ZnO to 3 to 4.7 parts H PO the zinc being present in amounts equivalent to from 0.026 to 0.21 gram ZnO per liter, the sodium acid pyrophosphate being present in an amount equivalent to from 1 to 6 grams per liter, the solution also containing manganese in proportions up to 0.17 gram Mn per liter.

5. A process of phosphatizing metals by plating on said metals zinc and iron pyrophosphates, which process comprises, contacting the metal to be phosphatized with a solution comprising sodium acid pyrophosphate, zinc and phosphoric acid, the phosphoric acid being present in proportion of 1 part ZnO to 3 to 4.7 parts H PO and the zinc being present in amounts between 0.026 and 2.5 grams per liter, the sodium acid pyrophosphate being present in amounts from 1 to 25 grams per liter said ingredients in the solution reacting to form zinc pyrophosphate and zinc acid phosphate, said solution being maintained at a temperature of from about 60 C. to about 82 C.

6. A powdered preparation for making a phosphatizing bath comprising, from about 0.026 to about 2.5 parts of zinc oxide, phosphoric acid, from about 1 to about 25 parts of sodium acid pyrophosphate and from about 1.2 to about 10 parts of sodium orthophosphate, the phosphoric acid being present in the proportion of 1 part ZnO to 3 to 4.7 parts H PO 7. A powdered preparation for making a phosphatizing solution comprising, from about 0.026 to about 2.5 parts of zinc oxide, phosphoric acid, from about 1 to about 25 parts of sodium acid pyrophosphate and from about 1.2 to about 10 parts of sodium orthophosphate,

part ZnO to 3 to 4.7 parts H PO the composition also including a minor proportion of manganese carbonate.

8. A powdered preparation for making a phosphatizing bath comprising, from about 0.026 to about 2.5 parts of zinc oxide, phosphoric acid, from about 1 to about 25 parts of sodium acid pyrophosphate and from about 1.2 to about 10 parts of sodium orthophosphate, the phosphoric acid being present in the proportion of 1 part ZnO to 3 to 4.7 parts H PO the composition also including a minor proportion of manganese carbonate and citric acid.

9. A powdered composition for producing a phosphatizing solution comprising from about 0.026 to about 2.5 parts of zinc oxide, phosphoric acid, from about 1 to about 25 parts of sodium acid pyrophosphate, from about 1.2 to about 10 parts of mono sodium phosphate, the phosphoric acid being present in the proportion of 1 part ZnO to 3 to 4.7 parts H PO and from about 0.9 to about 7.5 parts of hemi basic sodium phosphate.

10. A powdered composition for producing a phosphatizing solution comprising from about 0.026 to about 2.5 parts of zinc oxide, phosphoric acid, from about 1 to about 25 parts of sodium acid pyrophosphate, from about 1.2 to about 10 parts of mono sodium phosphate and hemi sodium phosphate, the phosphoric acid being present in the proportion of 1 part ZnO to 3 to 4.7 parts H PO the composition also including up to 0.17 parts of manganese carbonate.

11. A powdered composition for producing a phosphatizing solution comprising from about 0.026 to about 2.5 parts of zinc oxide, phosphoric acid, from about 1 to about 25 parts of sodium acid pyrophosphate, from about 1.2 to about 10 parts of mono sodium phosphate and hemi sodium phosphate, the phosphoric acid being present in the proportion of 1 part ZnO to 3 to 4.7 parts H PO the composition also including minor proportions of manganese carbonate and citric acid.

12. A powdered composition for producing a phosphatizing solution comprising from about 0.026 to about 2.5 parts of zinc oxide, phosphoric acid, from about 1 to about 25 parts of sodium acid pyrophosphate, from about 1.2 to about 10 parts of mono sodium phosphate and hemi sodium phosphate, the phosphoric acid being present in the proportion of 1 part ZnO to 3 to 4.7 parts H PO the composition also including minor proportions of manganese carbonate, citric acid and molybdic oxide.

13. A process of phosphatizing metals which comprises, contacting the metal with a phosphatizing solution comprising zinc, free phosphoric acid, sodium acid pyrophosphate, and a minor proportion of citric acid, the initial ratio of zinc to free phosphoric acid being in the proportion of from 1 part ZnO to 3 to 4.7 parts of H PO the zinc being present in an amount equivalent to 0.026 to 2.5 grams per liter, and the sodium acid pyrophosphate being present in amounts from 1 to 25 grams per liter.

14. A process of phosphatizing metals which comprises, contacting the metal with a phosphatizing solution comprising zinc, free phosphoric acid, sodium acid pyrophosphate, and manganese in proportions up to 0.17 grams Mn per liter, the initial ratio of zinc to tree phosphoric acid being in the proportion of from 1 part ZnO to 3 to 4.7 parts of H PO the zinc being present in an amount equivalent to 0.026 to 2.5 grams per liter, and the sodium acid pyrophosphate zeing present in amounts from 1 to 25 grams per liter.

15. A process of phosphatizing metals which comprises, contacting the metal with a phosphatizing solution comprising Zinc, free phosphoric acid, sodium acid pyrophosphate, a minor proportion of citric acid, and sufficient sodium orthophosphate to bring the pH of the phosphatizing solution within the range of 3 to 4, the inital ratio of zinc to free phosphoric acid being in the proportion of from 1 part ZnO to 3 to 4.7 parts of H PO the zinc being present in an amount equivalent to 0.026 2,298,280

to 2.5 grams per liter, and the sodium acid pyrophoshate 2,316,811.

being present in amounts from 1 to 25 grams per liter. 2,337,856

References Cited in the file of this patent 5 2,557,509

UNITED STATES PATENTS 2,067,007 Darsey Ian. 5, 1937 8 Clifiord et a1. Oct. 13, 1942 Romig Apr. '20, 1943 Rice et a1. Dec. 28, 1943 McBride Mar. 22, 1949 Miller June 19, 1951 

1. A PROCESS OF PHOSPHATIZING METALS WHICH COMPRISES, CONTACTING THE METAL WITH A PHOSPHATIZING SOLUTION COMPRISING ZINC, FREE PHOSPHORIC ACID AN SODIUM ACID PYROPHOSPHATE, THE INITIAL RATIO OF ZINC TO FREE PHOSPHORIC ACID BEING IN THE PROPORTION OF FROM 1 PART ZNO TO 3 TO 4.7 PARTS OF H3PO4, THE ZINC BEING PRESENT IN AN AMOUNT EQUIVALENT TO 0.026 TO 2.5 GRAMS PER LITER, AND THE SODIUM ACID PYROPHOSPHATE BEING PRESENT IN AN AMOUNTS FROM 1 TO 25 GRAMS PER LITER SAID INGREDIENTS IN THE SOLUTION REACTING TO FORM ZINC PYROPHOSPHATE AND ZINC ACID PHOSPHATE, SAID SOLUTION BEING MAINTAINED AT A TEMPERATURE OF FROM ABOUT 60*C. TO ABOUT 82*C. 